Given the major advances made with modern radar technologies regarding object detection, essentially two approaches have been developed to minimize detection of an aircraft. The first is use of stealth technologies to reduce the radar signature of an aircraft by such means as radar absorbent materials, non-metallic structures, shapes that deflect radar electromagnetic waves away from the radar source, and the like. However, such means can lead to aircraft that are costly to build as well as difficult to fly, for example, modern stealth aircraft are notoriously difficult to fly and require a high order of pilot assistance by onboard computer control and sensing systems.
Hence, a multitude of aircraft are in operational service which have minimal or no benefit of stealth technologies. For these aircraft, a second approach to minimizing detection by radar is low level flight. It is more difficult to detect an aircraft flying in low level flight, for example, on the horizon, at treetop level, etc., than it is to detect an aircraft flying in exposed flight against the broad expanse of sky. However, while low level flight can minimize detection of aircraft and accordingly reduce the ability for the enemy to engage the aircraft with weapons such as surface-to-air missiles, low level flight can place the aircraft within range of small arms fire, such as rifle fire. In view of flying in such a hostile environment, it would be beneficial for all on board the aircraft to identify that a projectile is proximate, is hitting, or has the potential to hit the aircraft and further for the source of the incoming projectiles to be determined. Such hostile engagement and according avoidance of small arms fire is particularly applicable to aircraft such as helicopters where the top speed of flight may not be as high as an aircraft such as a fighter jet, and also owing to a common operation of a helicopter being in stationary/near stationary flight, e.g., in a hover. By determining the helicopter is under enemy attack (even where the enemy aim is such that projectiles are not impacting the helicopter) the helicopter can be quickly maneuvered to a safer location and/or height, as well as engage the enemy with whatever weapons are available (e.g., onboard the helicopter) to the helicopter crew.
A conventional system for detecting a helicopter is under attack (e.g., by small arms fire) comprises a microphone array, a digital signal processor and a display system with audio warning signal. A microphone array forming the foundation of the system is typically designed with an array of omni directional microphones which can be employed to pick up acoustic signals from the firing source(s), where the acoustic signals can comprise of both shockwaves generated by the projectile (e.g., at supersonic velocity) and muzzle waves generated from the source muzzle owing to detonation of the explosive facilitating propulsion of the projectile. Utilization of the acoustic signals enables a shooter location to be estimated.
However, in various situations, the muzzle wave may not be detectable by the microphone array, and hence, in such situations, the shooter location has to be estimated from the shockwave alone. To facilitate shockwave detection only, a network of sensors (e.g., microphones) are required to achieve a reasonable degree of certainty regarding knowledge of the shooter location. However, for a flying platform, such as a helicopter, maintaining the array in a fairly fixed location with regard to the shooter location is not always feasible, with a resulting degradation in the level of certainty with which the location of the shooter can be established.
The omni-directional microphone or hydrophone utilized in such an array is typically broadband in nature to facilitate capture of the signature of the shockwave and muzzle wave as required for shooter location detection, false alarm control and signal classification. The broadband nature renders the array susceptible to receiving all other sounds in the vicinity of the array which can also lead to distortion of the acoustic signals of interest. Hence, use of such an array is problematic in a noisy environment such as on the airborne platform like helicopter which can be extremely noisy in operation, for example from the noise of the downdraught wind (wind noise) as well as mechanics of the rotary system (platform noise) powering the rotor blades.
Platform noise generated by aircraft, such as a helicopter, can be in the order of 145 dB. Noise of this magnitude will drown out all signals of interest (i.e., shockwaves and muzzle waves) rendering identification of the signals of interest an almost impossible task. Array processing techniques may be applied to place nulls into the direction(s) of the unwanted noises. However, if the front ends of the microphones' are already in a situation of being saturated by the noise, which is a likely scenario, it still may be impossible to extract the wanted signals by an array processing technique.
A vector sensor can be utilized in a position to render a null in the noise source direction. However vector sensors are extremely susceptible to wind noise, thus rendering them unsuitable for application in a moving platform such as a helicopter.
Wind noise can be even more devastating to the array, even when filtering of the wind noise is conducted. However for a flying platform, such as a helicopter, flight speed can be high accompanied by noise generated by the downward thrust of air from the rotors, leading to complete saturation of the microphone front ends.